Patent Application
20240066517
The invention relates to a plunger rod for a liquid transfer system, in particular a pipette or a multichannel bottom part for a pipette, to a method for producing such a plunger rod and to a liquid transfer system with such a plunger rod according to the preamble of the independent claims.
Pipettes are used in particular in a laboratory setting for controlled drawing up and delivery of liquids. To this end, a pipette tip with an upper opening is firmly clamped onto a pipette seat. The seat is preferably formed as a conical or cylindrical projection on the pipette body. The pipette tip can draw up and deliver liquid through a lower opening. Air displacement pipettes comprise a displacement device for air which is in communicative connection with the pipette tip through a hole in the seat. By way of the displacement device, an air cushion is moved such that liquid is aspirated into the pipette tip and expelled therefrom. To this end, the displacement device has a displacement chamber with a movable boundary element. The displacement device is usually a cylinder with a plunger rod movable therein. A sealing element is arranged on the plunger rod and seals the plunger rod relative to a cylinder surrounding the plunger rod.
Multichannel fluid transfer devices are in particular multichannel pipettes and multichannel dispensers, which are intended for the calibrated aspiration of liquids and/or subsequent delivery of the liquids into containers. While aspirating and dispensing the liquid, the multichannel fluid transfer device is held in an operator's hand. A multichannel fluid transfer device has a top part, in which are arranged the operating elements and the mechanical components or electronics needed to aspirate the liquid, and a multichannel bottom part with a plurality of parallel-arranged dispensing channels, which are designed to draw up and/or deliver a liquid.
The precision and accuracy of the volume that is aspirated depends on the ability of each displacement element to reproduce the same, simultaneous travel for a given volume setting. It has been observed, however, that when a multichannel pipette is in use, the plunger actuator has a tendency not to maintain orthogonality to the sliding direction during movement. This effect, known as tilting clearance, arises during the reciprocating motion of the plunger actuator and the displacement elements and results in a difference in the distance traveled by one displacement element relative to another displacement element, thus reducing the precision and accuracy of the aspirated volume among the plurality of displacement elements. This rake effect may be caused by excessive guide clearance between the plunger actuator and the plunger actuator guide, by an unsuitable shape or hardness of the restoring spring, which is used to raise the plunger actuator, by excessively low rigidity of the plunger actuator guide and by different levels of friction at the seals of the displacement elements. The displacement elements in each case comprise a plunger rod on which a sealing element is arranged.
DE 10 2006 031 460 B4 discloses a multichannel pipette for dispensing liquids which has a plunger and a plurality of receptacles arranged at one end of the plunger actuator rod for in each case one plunger head. A resilient element which assists in centering the plunger in the receptacle is arranged between the plungers of the individual channels of the multichannel pipette and the receptacle for the respective plunger head.
EP 2 633 915 B1 discloses a manual single channel air displacement pipette with a body with a cylindrical displacement chamber formed therein in which a plunger with a plunger rod and a seal for drawing up and subsequently delivering a liquid is displaceable.
A disadvantage of the solutions known from the prior art, however, is that the plunger rod takes the form of a one-piece injection molding. To enable simple demolding of the plunger rod from the injection mold, a mold parting line is generally provided along a longitudinal axis of the plunger rod, resulting in the plunger rod having mold-related parting line flash over its entire length. This parting line flash may be removed at least in part in a deflashing process subsequent to the injection molding process, for example by vibratory finishing, abrasive blasting or tumbling. This entails the risk of the mold-related parting line flash not being fully removed in the region of a sealing seat of the plunger rod and/or of residual particles from the deflashing process adhering to the plunger rod, possibly leading, during operation of the pipette or of the multichannel bottom part, to a lack of tightness in the region of a sealing element drawn onto the plunger rod because the sealing element does not lie completely tightly against the plunger rod or particles of the parting line flash get in between the sealing element and the sealing seat formed on the plunger rod. Such a lack of tightness can only be identified on the finished product during final tightness testing of the pipette or of the multichannel bottom part. Replacing the plunger rod requires a lot of effort, which is associated with high assembly costs. There is also a risk of a leak arising before the pipette reaches the intended end of its service life, which will result in a customer complaint.
The object of the invention is consequently to avoid leaks between a plunger rod and a sealing element arranged on the plunger rod and so to enhance the tightness and service life of a liquid transfer system, in particular of a pipette.
The object is achieved by a plunger rod for a liquid transfer system. The plunger rod in this case is of two- or multipart construction and has a first plunger rod component which may be brought into operative connection with an actuating device of the liquid dispensing system. The plunger rod further comprises a second plunger rod component different from the first plunger rod component, and a sealing seat for accommodating a sealing element for the plunger rod. A parting line is formed in this case between the first plunger rod component and the second plunger rod component in such a way that the sealing seat is free of parting line flash.
A multipart configuration of the plunger rod can prevent an injection mold parting line and the associated parting line flash from forming in the region of the sealing seat. In this way, a sealing element can be prevented from lying in a non-fluid-tight manner against the sealing seat of the plunger rod and causing a leak at this point, which would lead to labor-intensive and costly refinishing during assembly since such a leak is only identified during final tightness testing of the liquid transfer device. A potential source of risk in terms of wear to the sealing element, which shortens the service life of the sealing element and can have a negative effect on the life of the liquid transfer device, can moreover be eliminated.
The features set out in the dependent claims enable advantageous improvements and further developments of the plunger rod stated in the independent claim.
In a preferred configuration of the plunger rod, provision is made for the second plunger rod component to be arranged or configured concentrically to the first plunger rod component. To ensure tightness between the plunger rod and a cylinder surrounding the plunger rod, tight manufacturing tolerances are needed for the plunger rod. Differences in roundness may in particular lead to the sealing element sealing inadequately either relative to the plunger rod or to a cylinder wall of the cylinder. To be able to comply with these tight tolerances, it is advantageous for the second plunger rod component to be arranged or configured concentrically to the first plunger rod component.
In one advantageous configuration of the plunger rod, provision is made for the sealing seat for receiving the sealing element to be formed between the first plunger rod component and the second plunger rod component. This is a simple way of preventing a mold parting line from running through the sealing seat on the plunger rod. Instead, the division into two components allows demolding requiring no mold parting line in the region of the sealing seat and thus producing a parting line flash-free outcome. In this way, the risk of a potential leak due to the sealing element lying poorly against the sealing seat can be eliminated. Furthermore, the sealing element may be simply guided and held in position by such a configuration.
It is particularly preferable, in this respect, for the first plunger rod component to have a fastening mechanism for a plunger actuator of a pipette or a multichannel bottom part for a pipette at its end remote from the sealing seat. In particular in the case of multichannel bottom parts for pipettes, there is a need for a plurality of plunger rods to be guided in parallel and with a high level of accuracy. To this end, a “plunger actuator” is generally used, which engages in an end portion of the respective plunger rod and thus enables parallel displacement of multiple plunger rods, such that the same quantity of liquid is in each case drawn up into the various cylinders of the multichannel bottom part.
According to one advantageous configuration of the plunger rod, provision is made for the second plunger rod component to be configured as a plunger tip, which is arranged as an axial extension of the first plunger rod component. In particular in the case of small plungers, as are used for the individual channels of a multichannel bottom part to draw up liquid, small plunger rods with a small diameter are needed. It is precisely in the case of narrow plungers with a small diameter that it may be particularly advantageous to connect a plunger tip, which extends the first plunger rod component in the axial direction, with the first plunger rod component in order to form a sealing seat without parting line flash.
It is particularly preferable, in this respect, for the plunger tip to be materially bonded to the first plunger rod component by way of a two-component injection molding method. A two-component injection molding method should be understood in this context to mean a two-stage injection molding method in which the second plunger rod component is produced in the form of the plunger tip in a first injection molding method and the second plunger rod component is molded on in a second injection molding method. The same plastics or different plastics can be used for the two injection molding methods. For the purposes of this patent application, a two-component molding method should also be understood to mean the use of two identical plastic materials in a two-stage injection molding method. A material bond produced by way of a two-component injection molding method enables particularly precise centering of the plunger tip relative to the first plunger rod component. In this way, the roundness requirements for the sealing seat can be particularly precisely complied with and tightness against the plunger rod enhanced.
Alternatively, a further preferred configuration of the plunger rod provides for the second plunger rod component to be configured as a cover or a dish, which is connected interlockingly or force-lockingly with the first plunger rod component. It is precisely in the case of relatively large plunger rods, as are used for example in single channel pipettes, that it may be advantageous to use a plunger rod constructed of two or more components. In order to comparatively straightforwardly form a sealing seat on the plunger rod which is free of parting line flash, the plunger rod is formed in two parts from a substantially elongate and cylindrical first plunger rod component and a cover or dish arranged on this first plunger rod component. Demolding of the cover or dish from an injection mold is particularly simple due to the simple geometry, which may be configured in particular without undercuts. Furthermore, the first plunger rod component can be of comparatively simple construction and the inclined surfaces needed for demolding can be provided without them extending over a sealing seat of the plunger rod.
It is particularly preferable, in this respect, for the first plunger rod component to have a shaft, wherein a recess is formed in the shaft for form-locking connection of the cover or dish to the shaft by way of a latching connection. In order to straightforwardly position the dish or cover relative to the first plunger rod component and thus to form the sealing seat, it is advantageous when a recess for form-locking connection of the cover or dish to the first plunger rod component is formed in the shaft of the first plunger rod component.
In a further improvement of the plunger rod, provision is made for the cover or dish to have a central opening, wherein at least one latching lug is formed at the central opening for latching into the recess in the shaft of the first plunger rod component. The dish or cover can be pushed simply and with application of little force by way of a central opening onto the first plunger rod component. The latching lug allows the dish or cover to be latched in the recess of the first plunger rod component, so enabling locking of the cover or dish in a defined position relative to the first plunger rod component. The latching lug on the dish or cover and the recess in the shaft may in particular be incorporated into the respective injection mold and thus produced in a substantially cost-neutral manner.
In a preferred configuration of the plunger rod, provision is made for the first plunger rod component and the second plunger rod component to be made of a plastics material, preferably of a thermoplastic, particularly preferably of polyoxymethylene (POM), polyetherether ketone (PEEK), polyarylether ketone (PAEK), polyamide (PA) or polyphenylene sulfide (PPS). Plastic materials are particularly suitable for use in laboratory devices such as pipettes due to their high level of chemical resistance and their low weight. Due to the stringent requirements regarding the geometry and tolerances of the two components of the plunger rod, thermoplastics are particularly suitable for reliably providing the required tolerances regarding length, diameter, position and component shrinkage after the injection molding process.
It is particularly preferable, in this respect, for the first plunger rod component and the second plunger rod component to be made from the same plastics material. In this way, it can be ensured that the two components have the same shrinkage behavior after the injection molding method and the same thermal expansion. In this way, it can be ensured that the components of the plunger rod do not move relative to one another or loosen due to repeated heating and cooling, for example during autoclaving of the liquid transfer device.
Alternatively, the first plunger rod component may also be constructed from a first plastics material, in particular a first thermoplastics and the second plunger rod component from a second plastics material, in particular a second thermoplastic, different from the first plastics material. In this way, the hardness and wear resistance of the components can be adapted to the respective requirements of the components of the plunger rod.
A further sub-aspect of the invention relates to a method for producing such a plunger rod. The method here comprises the following steps:
Using the described method, a plunger rod for a liquid transfer device may be produced which is formed free of parting line flash in the region of the sealing seat and thus enables improved sealing against the shaft of the plunger rod. In this way, a sealing element can be prevented from lying in a non-fluid-tight manner against the sealing seat of the plunger rod and causing a leak at this point, which would lead to labor-intensive and costly refinishing during assembly since such a leak is only identified during final tightness testing of the liquid transfer device. A potential source of risk in terms of wear to the sealing element, which shortens the service life of the sealing element and can have a negative effect on the life of the liquid transfer device, can moreover be eliminated.
In one advantageous configuration of the method, provision is made for the plunger rod to be produced using a two-component injection molding method in which the second plunger rod component is produced in a first injection molding process and this second plunger rod component is molded around in sections in a second injection molding process by the first plunger rod component, such that the second plunger rod component is concentric to the first plunger rod component and extends in the axial direction as an extension of the first plunger rod component. A material bond produced by way of a two-component injection molding method enables particularly precise centering of the second plunger rod component relative to the first plunger rod component. In this way, the roundness requirements for the sealing seat can be particularly precisely complied with and tightness against the plunger rod enhanced.
It is particularly preferable, in this respect, for the second plunger rod component to be inserted as an insert into the injection mold for the second injection molding process to produce the first plunger rod component. In this way, particularly tight manufacturing tolerances can be achieved regarding position, roundness and concentricity between the first plunger rod component and the second plunger rod component.
In an alternative embodiment of the method, provision is made for the second plunger rod component to be pulled, by way of a central opening in the second plunger rod component, onto the first plunger rod component and connected force-lockingly or interlockingly therewith. In this way, a plunger rod can be produced which may be assembled simply from two components and is constructed such that the region of the sealing seat is free of a potential source of risk in the form of parting line flash formed by an injection mold parting line.
According to a further alternative embodiment of the method, provision is made for the first plunger rod component and the second plunger rod component to be materially bonded together using spin welding. A spin welding method is a suitable method, in particular for relatively large plungers, for bonding the two plunger rod components materially together with tight manufacturing tolerances. Provision is made, in this case, for a weld from the spin welding method to be located away from the sealing seat, such that the sealing seat is free of weld residues or accumulated material resulting from the welding method.
A further sub-aspect of the invention relates to a liquid transfer device for drawing up and subsequent delivery of a liquid with such a plunger rod. To improve the service life and tightness of a liquid transfer device and avoid potential refinishing during assembly as a result of leaks, it is advantageous to use a plunger rod according to the invention in a liquid transfer device. The liquid transfer device may in particular be a pipette, in particular a manual or an electronic, hand-held pipette. Alternatively or in addition, the liquid transfer device may also be or comprise a multichannel bottom part for a pipette. In a further alternative embodiment, the liquid transfer device may comprise a dispensing tool for an automatic dispenser, in particular an automatic dispenser used in laboratory technology, particularly preferably an automatic pipette.
The different embodiments of the invention set out in this application are advantageously combinable with one another unless otherwise specified.
The invention is described below with reference to exemplary embodiments and on the basis of the associated drawings, in which:
The second plunger rod component 14 is configured as a cover 34 or a dish 36, which is arranged concentrically to the first plunger rod component 12 and in particular is pulled up over the shaft 38 onto the first plunger rod component 12. To this end, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 22193005.0 | Aug 2022 | EP | regional |
